The present invention relates in general to an AM/FM broadcast reception antenna for a vehicle, and more specifically to a multiband antenna that may be located around the perimeter of a panel of window glass.
As an alternative to standard whip antennas, surface conformal antennas are often used on automobiles to obtain advantages of better resistance to damage, elimination of wind noise, and less negative impact on styling. An on-glass antenna is one type of conformal antenna. It is formed using known techniques such as silk-screen printing operations for depositing a silver ceramic paste to form antenna conductors, antenna feedlines, and antenna terminals. After depositing the silver ceramic paste, the glass sheet is placed on a fixture and heated in an oven to a temperature adequate to bond the silver ceramic paste to the glass sheet. Details on forming conductive segments and terminals on a glass sheet are provided in commonly assigned U.S. Pat. Nos. 4,246,467 and 4,388,522, incorporated herein by reference.
Despite the potential advantages of on-glass antennas, their acceptance has been slow due to the difficulty of meeting adequate performance standards with on-glass antennas. Many different on-glass antenna patterns have been tried in an effort to improve antenna performance. Many patterns intrude upon the visible area of a window which has an undesirable effect on styling appearance.
In the case of multiband antennas intended to receive both AM and FM signals via a single antenna feed line, incompatible antenna requirements have compromised the effectiveness of on-glass antennas. For example, an AM antenna needs to be as long as possible in order to maximize signal reception (because the length of an AM antenna is much shorter than the AM wavelength). In contrast, the length of an FM antenna is comparable to an FM wavelength and, therefore, the antenna length must be selected to be resonant at the FM frequencies of interest (e.g., quarter-wave, half-wave, or full-wave). Thus, as antenna length is increased in an effort to improve AM reception, FM reception suffers if the antenna length is increased beyond a resonant length for the FM frequencies.
In order to provide acceptable performance, an on-glass antenna must also provide sufficient bandwidth (i.e., gain across the frequency band of interest). Co-pending application Ser. No. 08/379,409, filed Jan. 27, 1995, entitled "VEHICLE WINDOW GLASS ANTENNA ARRANGEMENT", hereby incorporated by reference, discloses that the diameter of an antenna conductor can be increased in order to increase bandwidth. Thus, the width of the antenna conductor traces on the glass can be widened to increase bandwidth; however, visibility through the window is decreased.
The competing requirements of different bands can be addressed by adopting separate antennas or antenna feeds. Multiple coaxial transmission lines running from the antenna to the receiver can be avoided by combining the separate antenna signals using an electrical network. Such a network, however, involves the added complexity and expense of a separate module. In order to limit the complexity and expense of an on-glass antenna system, the number of antenna feeds needs to be kept to a minimum.